Restriction indicators are commonly used to indicate when it is recommended to service the air cleaner system of an internal combustion engine. Engine air cleaner systems usually use barrier filtration media that requires periodic servicing to ensue that adequate air flow is provided to the engine. Restriction indicators typically are connected to the air outlet of the air cleaner system upstream of the intake of the engine. Many mechanical restriction indicators employ a bellows-type diaphragm vented to the atmosphere, which is resisted from expanding by a compression spring. As the air cleaner system becomes more contaminated, the differential pressure between the “atmospheric”/high pressure side and the “negative”/low pressure side of the restriction indicator increases, thereby causing the diaphragm to expand. When the differential pressure has reached a certain level, the diaphragm typically becomes locked in an expanded position, and a constant warning signal is provided to the user that the air cleaner system is recommended to be serviced. An example of such prior art restriction indicators is shown in U.S. Pat. No. 3,443,365.
A problem associated with restriction indicators is providing a locking mechanism which will consistently lock and release after repeated use. In many designs, the locking mechanism includes a resettable plunger which engages a latch to lock the diaphragm in an expanded warning position. An example of this is shown by the '365 patent, in which the plunger is bent to engage the latch off-center. Another example is shown by U.S. Pat. No. 4,445,456 which discloses a restriction indicator having a ratchet that swings through an arc and interlocks with a floating indicator. An inherent disadvantage of this indicator is that side loads from the ratchet can cause canting, misalignment, friction and binding of the floating indicator.